The present invention relates to electrooptic devices whose active material is a superlattice material.
It is known that electromagnetic wave propagation in an isotropic medium causes the induced polarization to be parallel to the electric field and to be related to it by a (scalar) factor that is independent of the direction along which the field is applied. This situation does not apply in the case of anisotropic crystals. Since the crystal is made up of a regular periodic array of atoms (or ions), the induced polarization will depend, both in its magnitude and direction, on the direction of the propagating field. Instead of a simple relation linking the induced polarization p, and the electric field, E, a 3.times.3 array called the electric susceptibility tensor relates p to E. One of the most important consequences of anisotropic dielectric crystals is the phenomenon of birefringence in which the phase velocity of an optical beam propagating in the crystal depends on the direction of polarization of its electric field vector.
It is possible to effect a change in the index of refraction of anisotropic crystals which is proportional to an applied electric field. This is the linear electrooptic effect. It affords a convenient and widely used means of controlling the intensity or phase of propagating optical radiation in the crystal by the applied electric field. This modulation is used in an ever expanding number of applications including:
the impression of information onto optical beams, Q-switching of lasers for generation of giant optical pulses, mode locking, and optical beam deflection.
The linear electrooptic effect is the change in the indices of the ordinary and extraordinary rays that is caused by and is proportional to an applied electric field. This effect exists only in crystals that do not possess inversion symmetry. The division of all crystal classes into those that do and those that do not possess an inversion symmetry is an elementary consideration in crystallography and this information is widely tabulated. Up to the present time, crystalline materials have been used almost exclusively in electrooptic devices. This is because non-zero electrooptic coefficients occur only in materials which lack a center of symmetry. In the present invention, semiconductor and insulator superlattice materials are used as the active material for electrooptic devices. The superlattice materials are suitable because they lack symmetry due to the presence of large built-in electric fields in each layer.
These superlattice materials have electrooptic coefficients comparable to the best crystalline materials available. They also have the considerable advantage that they can be deposited at a relatively low temperature onto a wide variety of substrates. They can be used for both longitudinal and transverse optical modulators.